This invention relates to feedforward amplifier circuits.
Feedforward amplifier circuits are well known in the art, an example of which may be viewed by referring to FIG. 1, a schematic diagram of a prior art, wide-band, feedforward amplifier circuit. An input signal such as a radio frequency signal is applied to the feedforward amplifier circuit at terminal 1 which is connected to the directional coupler 3 at terminal A. In some embodiments, the signal that is present on the output terminal B of the directional coupler 3 is attenuated by the coupler by a selected amount such as 10 or 30 DB. Terminal A' is the remaining output terminal and the signal on Terminal A' is supplied to the power amplifier 5 which in the case of most feedforward circuits is a wide-band power amplifier. The directional coupler 3, due to its inherent capabilities, essentially samples the signal that is present on terminal A and provides a sample signal to the delay line 6 and the comparator 9. The comparator 9 compares the delayed sampled signal that is present on terminal D with an amplified representation of that signal that has been amplified by the amplifier 5 and sampled by the directional coupler 7 which is similar to the directional coupler 3 and obtains the difference of the signal that is on terminal E of the directional coupler 7 and the delayed signal that was delayed by the delay line 6 that is present on terminal D, the negative terminal of the comparator 9. The difference that is on the .DELTA. terminal of the comparator 9 is amplified by the error amplifier 13 and applied to the terminal H of the directional coupler 15 which is used as a recombiner for recombining with the signal that is present on the terminal G the signal that is present on terminal H. The signal present on terminal G is the amplifier signal that was amplified by amplifier 5, sampled by directional coupler 7 and delayed by delay 11. The recombined output signal is present on terminal I of the directional coupler 15 and is connected by way of terminal 2 to load 17.
The attenuation factors and delay characteristics of the directional couplers 3 and 7 and delay 6 are selected such that the amplitude and phase of the input signal components on the + and - terminals of the comparator 9 are equal, rendering only distortion components at the .DELTA. terminal of comparator 9. Similarly, the attenuation factors and delay characteristics of the directional couplers 7 and 15 and delay 11 are selected to completely null the distortion components introduced by amplifier 5 and amplified by the error amplifier 13 so that the output signal at Terminal I does not have any distortion components as a result of amplication by the amplifier 5.
The mathematical derivation of the operation of the prior art circuit is provided in the Table of Equations by Equations 1 through 12. The voltage signals at the terminals are given the terminal letter as a subscript. The input signal that is applied to terminal 1 and terminal A is denoted V.sub.A and is mathematically represented in Equation 1. K is a constant and .omega. represents the radian frequency of the signal that is applied to terminal 1. The signal that is present on terminal B of the directional coupler 3 is represented by Equation 2. In this equation, .alpha..sub.1 is the attenuation factor introduced to the circuit by the directional coupler 3. In the equation representing the amplified signal V.sub.C, equation 3, G.sub.1 is the gain of the amplifier 5, and has included therein the equivalent distortion voltage D.sub.V.sbsb.1 which represents the distortion components injected into the system by the power amplifier 5 and T.sub.1 is the delay through amplifier 5. The delayed signal that is applied to the summing device 9 on the negative terminal D is represented by Equation 4 with .alpha..sub.3 being the attenuation provided by the delay 6 and T.sub.2 being the time delay provided by the delay 6. The sample of the amplified signal that is present on terminal E of the directional coupler 7 is provided by Equation 5 where .alpha..sub.2 is the attenuation factor of the directional coupler 7. The signal that is applied to the error amplifier 13 is represented by Equation 6 with .alpha..sub.5 being the attenuation of the comparator 9. The expansion of Equation 6 is provided in Equation 7.
The recombination of the signals as mentioned earlier is provided by the directional coupler 15. Equation 8 is the mathematical representation of the signal that is present on terminal G where an additional time delay T.sub.3 and attenuation .alpha..sub.4 is provided as a result of signal propogation through the delay 11.
The output of the error amplifier is represented by Equation 9 which is the difference between Equations 4 and 5 amplified by the error amplifier 13 with the G.sub.2 being the gain of amplifier 13 and T.sub.4 being the propogation delay of the signal by amplifier 13, and D.sub.V.sbsb.2 being the distortion components generated by amplifier 13. The mathematical recombination of the signal is provided by Equation 10 with V.sub.I being the signal that is present on terminal I of the directional coupler 15 and .alpha..sub.6 is the attenuation factor injected on the signal by the directional coupler 15. Equation 11 is the expansion of equation 10.
The selection of the directional couplers and delay lines as was discussed above are such that T.sub.3 =T.sub.4 and .sqroot..alpha..sub.4 =.sqroot..alpha..sub.2 .alpha..sub.5 G.sub.2 then equation 11 may be simplified so that the distortion from amplifier 5, D.sub.V.sbsb.1 is cancelled and equation 12 represents the output signal at Terminal I.
It should be noted that the distortion component from the error amplifier D.sub.V.sbsb.2 is considerably less than the distortion level of power amplifier 5. In the situation where D.sub.V.sbsb.2 is beyond acceptable limits than prior art circuits are known in which feedforwarding techniques are used to reduce the D.sub.V.sbsb.2 component.
In theory, an arbitrary high amount of correction can be applied using the above described feedforward system. The phase and gain matching requirements for achieving and maintaining a high level of distortion is generally prohibitive. The prior art has used multi-stage feedforward corrections where there is feedforward correction of the error amplifier 13. It is obvious that this technique may be extrapolated on down through multi-stages. However, the cost becomes prohibitive when multi-stage feedforward circuits are used.